The invention relates to a method for measuring properties of a paper web, in which method at least one property of the paper web is measured with at least one measuring means that transmits a measuring beam at least on one measuring channel such that at least two locations in the cross direction of the paper web are measured simultaneously.
The invention further relates to an apparatus for measuring properties of a paper web, the apparatus comprising at least one measuring means having means for transmitting a measuring beam at least on one measuring channel, whereby the apparatus is arranged to measure at least one property of the paper web by measuring at least on two adjacent measuring channels in the cross direction of the paper web simultaneously.
It is known to measure properties of a moving paper web with a measuring device such that a measuring point of a measuring sensor traverses in the cross direction of the paper web. The measuring sensor is generally secured to a measuring bar positioned across the paper web. It is also known to use so-called optical traversing in measuring the properties of the paper web as disclosed in U.S. Pat. No. 5,073,712. In this method, a measuring sensor is fixedly mounted above the web and a measuring beam to be transmitted from the sensor traverses the web in the cross direction. Calibration of these measuring devices is carried out in such a way, for instance, that a reference sample is placed e.g. at the edge of the paper web, outside the web, and the measuring device measures the properties of said reference sample at suitable intervals, and on the basis thereof, calibrates the measuring means in a manner known per se. However, in the solution concerned, the measuring device measures the paper web diagonally, whereby measuring results will not be obtained from adjacent locations, for instance. The measuring method is also relatively slow.
To speed up a measurement and to obtain adjacent measuring results, it is known to use solutions, in which the paper web properties are measured simultaneously at adjacent measuring points. Solution of this kind are disclosed, for instance, in U.S. Pat. Nos. 4,565,444 and 4,801,809. Further, the publication by Pertti Puumalainen xe2x80x9cPaperikoneen CD-mittausten tulevaisuudennxc3xa4kymxc3xa4t, Paperirataa on-line mittaavat laitteet ja niihin liittyvxc3xa4t s{umlaut over (aa)}dxc3x6t, Feb. 24-25, 1998, Lappeenrantaxe2x80x9d(The prospects of paper machine CD measurements, Devices measuring the paper web online and adjustments related thereto, Feb. 24-25, 1998, Lappeenranta) sets forth a solution in which a plurality of measuring devices are adjacently positioned and each measuring device is moved back and forth for a portion of the paper web in the cross direction. Thus each sensor analyzes a small portion of the paper web width. However, calibration is very difficult in these solutions. In the above-mentioned publication by Puumalainen, a reference sample is placed above each measuring device, and for calibration, the measuring bar, onto which the measuring devices are placed, is turned upside down such that each measuring device then measures the values of the reference sample locating in front of the measuring device concerned. However, a problem with this solution is that various reference samples may originally be different or they may become different due to aging or various outside influences, such as fouling, and consequently the measuring devices are calibrated onto different levels, i.e. their readings become different. The structure of the solution in question is also very complicated and hence cumbersome and expensive.
The object of the present invention is to provide a method and an apparatus in which the above drawbacks are eliminated. A further object is to provide a method and an apparatus by means of which measuring of the properties of a moving paper web is fast and the measuring results are accurate and reliable.
The method of the invention is characterized in that the measurement is carried out as a reflection measurement and that the channels measuring different locations are calibrated by moving at least one reference sample across the path of the measuring beams in the cross direction of the paper web.
Further, the apparatus of the invention is characterized in that the measuring means comprises a transmitter and a receiver which are arranged on the same side of the paper web and that the apparatus comprises at least one reference sample that is movable across the path of the measuring beams in the cross direction of the paper web for calibrating the apparatus.
The basic idea of the invention is to measure at least one property of the paper web by measuring it at least at two locations in the cross direction of the paper web simultaneously and to calibrate the measuring means measuring different locations by moving at least one reference sample across the path of the measuring beams measuring different points in the cross direction of the paper web.
The invention has an advantage that the measuring channels of the measuring means can be calibrated or standardized on the same level simply and efficiently. The solution is very reliable and it improves the reliability and usability of the measurements considerably. A further advantage is that calibration can be performed while the web measurement is underway, because the calibration does not interfere with the measuring throughout the entire width of the paper web, since the reference sample is so small that in calibration it only covers the path of one or some of the measuring beams.
In the present specification, the term xe2x80x98paperxe2x80x99 refers to paper board and tissue, in addition to paper.
In the present specification, calibration refers to defining a quantity for a property of paper that is actually measured (temperature, etc.) The means for measuring the property must be properly calibrated so as to indicate the correct value of a stimulus measured. Thus all calibrated measuring means of the same type indicate the same measured value for the same measured stimulus.
However, several gauges are constructed such that a second property that has not been measured directly will be inferred on the basis of a first property by utilizing the correlation between the properties. For instance, several meters used in the paper industry direct the stimulus, such as radiant energy or a particle beam, at the paper whose properties are measured and then measure the modulated radiant flux or particle flux emitted by the paper. The mathematical relation describing the correlation between the properties obtained by these measurements is used for calculating the second property on the basis of the first property. The formula and parameters of this relation have to be previously known or predetermined. In some cases, the second property can be inferred on the basis of several measured properties by using a multivariable relation.
Because the strength or other properties of the source of stimulus may vary in various gauges or even in the same gauge with time, the correlation between the calibrated measurement and the property correlating therewith may vary in different gauges and at different times in the same gauge. Likewise, the correlation between the measured property and the correlating property may vary due to changes in other unmeasured properties. For instance, the correlation between microwave backscatter and sample moisture content changes if the sample contains carbon black.
Standardization is used for compensating for differences and changes in the stimulus or correlation. Standardization is also a means of calibration when the differences appearing in the stimulus or correlation are known or they are known to be insignificant.
In standardization, a property of at least one reference sample, whose one other property is known, is measured and a parameter representing the relation between the known property and the measured property is calculated for measuring means. A plurality of reference samples, one other property of each being known, are preferably used. If a plurality of reference samples, whose other known properties have different values, are used, it is possible to calculate a plurality of parameters for the relation. Thus, statistical methods, such as the method of least squares, can be used for calculating the most suitable parameters. By means of statistical methods, it is also possible to select the relation formula, in addition to other parameters.
Calibration is thus applied to properties that are measured directly and in connection wherewith a drift in the calibration of the measuring means is corrected. These properties include e.g. temperature and thickness of paper. Standardization is applied to properties that are measured indirectly and in connection wherewith one or more of the following features are simultaneously compensated for: i) differences in correlation between measured and inferred properties, ii) differences appearing in the stimulus used, iii) drift in the calibration of the measuring means. These properties include e.g. basis weight, moisture, ash content, colour, etc. For the sake of clarity, in the present specification, the term xe2x80x98calibrationxe2x80x99 also refers to standardization, in addition to calibration.